For large-scale photovoltaic power plant or distributed grid-connected power generation system, an inverter system performs an inversion on direct current energy of a photovoltaic array and then feeds it to a medium and high voltage power grid via a boosting transformer. When the light is weak, such as in the night or a rainy day, for a conventional photovoltaic grid-connected power generation system, only the inverter system is in standby mode, the boosting transformer is always connected with the medium and high voltage power grid, and in this case the boosting transformer generates no-load loss when the system is in standby mode.
In order to reduce the power loss of the whole power generation system, a medium and high voltage grid-connected technique is provided in the conventional art. Using conventional techniques, the boosting transformer serves as a part of the inverter system. The output terminal of the boosting transformer is connected with the medium and high voltage power grid via a high voltage contactor or a tap switch. And the high voltage contactor or the tap switch is controlled by a controlling unit of the inverter system so as to realize connecting and disconnecting of the medium and high voltage power grid. When the grid-connected system is in standby mode, the connection between the boosting transformer and the medium and high voltage power grid is cut off, to reduce no-load loss of the boosting transformer and improve the overall efficiency of the system. Before connecting to the medium and high voltage power grid, an excitation is performed on the boosting transformer with energy at a direct current side of an inverter unit of the inverter system, to generate a voltage of a same amplitude and a same phase with the medium and high voltage power grid. In this way, the impact on the boosting transformer and the high voltage device is reduced in the moment that the power grid is connected, and the critical life is improved.
However in the conventional art, the inverter system needs three connecting lines to collect the output voltage of the boosting transformer, and needs three more connecting lines to collect a voltage of the medium and high voltage power grid. When voltages on both sides of the high voltage contactor or the tap switch are same, that is, after the excitation is successful, the power grid input is connected by closing the high voltage contactor or the tap switch. The system requires six connecting lines to collect the voltages on both sides of the high voltage contactor or the tap switch, which makes the system wiring complex. In addition, the medium and high voltage power grid sampling is introduced in the inverter system, which reduces the isolation performance of the system and causes security risks.